A predictive model for microstructure evolution in metallic multilayers with immiscible constituents

Abstract

Focussing on the thermal stability of layered structures, we developed a predictive model to study the microstructure evolution of metallic multilayers with different morphologies including aligned and classical staggered grain geometries. We found that the zig-zag microstructure experimentally observed in multilayers forms when grains in each upper layer have a relative shift less than half the in-plane grain size to the lower layer. During this formation process, the non-equilibrium triple junctions move, driven by the imbalance of tensions of interphase and grain boundaries, corresponding to an extension to classical grooving theory. Numerical simulations show that a finite mobility of the triple junction can effectively impede the development of grooves, suggesting that the classical t1/4 dependence of groove depth with the assumption of an infinite triple junction mobility might be questionable at low temperatures to predict the time to pinch-off. Further, a map for the stability of layered structure in Cu/Nb system is developed in terms of the aspect ratio of grain dimensions and the ratio of the distance between two nearest triple junctions to the in-plane grain size. A criterion for this stability is also proposed for multilayers with similar grain boundary energies based on simplified geometrical consideration. Both the map and the simple criterion are in good agreement with the experiments for Cu/Nb multilayers.